1. Technical Field of the Invention
The invention relates to a corrosion-resisting, self-tapping screw.
2. Prior Art
A screw is known from DE 44 17 921 A1. It consists of a first screw member of corrosion-resisting steel and of a second screw member of hardened steel which is attached to the end of the first screw member. The two screw members are non-positively joined to each other, having a thread on their circumference. This design has the drawback that the connection between the first and the second screw member is destroyed substantially perpendicularly to the central longitudinal axis in the case of high screw-in forces acting on the thread.
WO 96/16274 teaches a self-cutting screw of the generic type. It has a cylindrical core with an axial groove provided therein, the depth of which reaches as far as to the central longitudinal axis, in some embodiments passing entirely through the core. A flat elongated insertion is inserted in the groove. The thread of the screw is designed so that the thread, which forms one piece with the core, blends harmoniously with the thread profile along the insertion. Manufacture of the screw is such that a pre-punched, flat insertion of durable steel is inserted and fixed in a substantially cylindrical screw shaft of stainless steel with an axial groove. Then the thread is formed for instance by rolling, which is followed by the hardening of the insertion. The known screw has numerous drawbacks. Since the depth of the axial groove reaches as far as to, and in part beyond, the central longitudinal axis, the core is partially severed thereby, its screw-in stability thus being strongly impaired. In particular concrete screws, when screwed in, require very high torques which act on the core. Furthermore, hardened cutting elements may be produced in the thread only along a single or maximally along two opposite surface lines, further severing of the core, for instance by grooves that are offset from each other by 120xc2x0, not being possible. Even in the case of two opposite grooves, the core is nearly completely severed so that it is strongly weakened. Moreover, the thread profile along the insertion blends harmoniously with the thread that forms a single piece with the core i.e., there are no cutting heads, in the proper sense of the word, projecting over the thread and possessing an increased cutting function which is needed especially for concrete.
U.S. Pat. No. 5,385,439 teaches a thread-forming screw. Along a smaller part of its thread, this screw has forming elements which stand out radially from the thread. The forming elements are one-piece with the core and the rest of the thread. A drawback resides in that, if the forming elements are to have sufficient hardness for cutting stone, the screw is not corrosion-resisting, because only non-corrosion-resisting steels can be hardened. On the other hand, non-corrosion-resisting screws do not comply with the requirements, set by safety standards, for the durability of concrete which is for instance exposed to humidity.
It is an object of the invention to improve a screw of the generic type so that its stability is ensured even in the case of important screw-in forces.
The gist of the invention resides in providing cutting elements with cutting heads of a hardened metal in a screw which consists substantially of a corrosion-resisting metal, the cutting heads being disposed within the thread and the cutting elements being inserted in the core in tilt-resisting manner. In this way, the cutting heads are capable of being loaded even by a high frictional forces on the wall of the drill hole, while a predominant portion of the thread is simultaneously corrosion-resisting and cannot be damaged in the long run.